The present invention relates to electrochemical conversion cells, commonly referred to as fuel cells, which produce electrical energy by processing first and second reactants. For example, electrical energy can be generated in a fuel cell through the reduction of an oxygen-containing gas and the oxidation of a hydrogenous gas. By way of illustration and not limitation, a typical cell comprises a membrane electrode assembly (MEA) positioned between a pair of flowfields accommodating respective ones of the reactants. More specifically, a cathode flowfield plate and an anode flowfield plate can be positioned on opposite sides of the MEA. The voltage provided by a single cell unit is typically too small for useful application so it is common to arrange a plurality of cells in a conductively coupled “stack” to increase the electrical output of the electrochemical conversion assembly.
The MEA typically comprises a proton exchange membrane separating an anode layer and a cathode layer of the MEA. The MEA is typically characterized by enhanced proton conductivity under wet conditions. For the purpose of describing the context of the present invention, it is noted that the general configuration and operation of fuel cells and fuel cell stacks is beyond the scope of the present invention. Regarding the general configuration and operation of fuel cells and fuel cell stacks, applicants refer to the vast collection of teachings covering the manner in which fuel cell “stacks” and the various components of the stack are configured. For example, a plurality of U.S. patents and published applications relate directly to fuel cell configurations and corresponding methods of operation. More specifically, FIGS. 1 and 2 of U.S. Pat. No. 6,974,648 and the accompanying text present a detailed illustration of the components of one type of fuel cell stack and this particular subject matter is expressly incorporated herein by reference.
FIG. 1 illustrates the regions of the anode side bipolar plate 5 (the cathode side would have similar regions). The anode side of the bipolar plate includes an active area 10, feed regions 15, tunnel regions 20, and manifolds 25. Liquid water accumulates in the anode (and cathode) outlet areas of bipolar plates during fuel cell operation. The water needs to be purged prior to stack shutdown to prevent the water from freezing and blocking the anode flow (anode starvation) upon subsequent stack startup at sub-freezing temperatures.